[dmitrygr]

Take a look at this: https://en.wikipedia.org/wiki/Off-the-Record_Messaging

It is designed for that, and more. In fact, it does not leave the theoretical "anyone could fake the logs", they created a tool to do so, so that you do not need an expert witness to explain to court that someone could doctor the logs - a tool exists for it. On purpose. They call it Deniable authentication:

> Messages in a conversation do not have digital signatures, and after a conversation is complete, anyone is able to forge a message to appear to have come from one of the participants in the conversation, assuring that it is impossible to prove that a specific message came from a specific person. Within the conversation the recipient can be sure that a message is coming from the person they have identified.